Workers at the Cadbury's factory at Keynsham, which is set to close, are demonstrating as part of their campaign to save 500 jobs at the site.
The closure will be phased in from 2009
The union Unite said its workers did not accept the company decision to shut the site near Bristol.
Unite said the factory was "profitable and productive" and there was "widespread anger" in the community.
Workers from the Cadbury's plant at Bournville in Birmingham also took part in Tuesday's protest.
It is thought some 200 people protested.
Staff in Keynsham were told during an emergency meeting last week that the phased closure would begin in 2009 and be finished by 2010.
The Keynsham production will be moved to factories in Poland and Bournville as part of a wider cost reduction plan announced in June.
Cadbury said its other two UK chocolate manufacturing sites, at Chirk in North Wales and Leominster in Herefordshire, would be "unaffected" by the plans.
Steve Preddy, regional industrial organiser for Unite, told the Birmingham crowd: "The announcement by Cadbury last week is nothing short of a betrayal of the work force in Keynsham.
"Unite will do everything possible to persuade Cadbury that they should reverse this appalling decision."
Dawn Naish, 41, from Barr's Court, Bristol, said her family would be devastated by the closure.
Mrs Naish, who has worked there for 17 years, said: "I'm the main bread winner in the family. "I've got two young children aged 10 and six. The closure of the factory will have an absolutely devastating affect on us."